In a perfect world, the audience should be able to clearly see everyone on stage all the time.
It’s not a perfect world, but here are some guidelines.
If you’re doing a large musical with chorus people in the background, it’s okay for the leads to stand in front of them. But the primary characters in crowd scenes should all be visible.
It’s okay to cross in front of someone when they aren’t speaking as long as you do it reasonably quickly and, heaven forbid, don’t stop in front of them.
Don’t forget the people sitting in the front row against the walls. They need to be able to see you, too. If you are upstage of other actors, pay attention during each performance so that you can adjust your position if necessary to give those audience members a glimpse of your face. Similarly, if you are the downstage actor, be courteous to those upstage and adjust your own position if you are aware that you are blocking them. Play with it in rehearsals so you will know what kind of flexibility you’ve got on performance nights without having to stare at the end seats to figure out if you need to move.
If you find that you are positioned on one side of the stage and looking toward the other side of the theater for a prolonged period of time, find reasons to turn your head in the other direction with some frequency. The audience on your side of the theater deserves more than just your profile, too.
And lastly, remember that when you’re backstage, be careful about peeking through the curtains to watch the action on stage. If you can see the audience, they can see you!